crypto/x509: root_cgo_darwin and root_nocgo_darwin omit some system certs [1.11 backport]

[gopherbot]

@FiloSottile requested issue #24652 to be considered for backport to the next 1.10 minor release.

There are multiple issues with our macOS root discovery.

The cgo path is unaware of defaults, documented at https://developer.apple.com/documentation/security/1400261-sectrustsettingscopytrustsetting, so it will omit the following certificate.

Cert 1: mkcert development CA
   Number of trust settings : 0

CL 104735 is an incomplete fix, because if trustSettings are present but don't have a kSecTrustSettingsResult value, it defaults to trustRoot. So it will omit the following certificate.

Cert 1: mkcert development CA
   Number of trust settings : 1
   Trust Setting 0:
      Policy OID            : SSL

The nocgo path, on the other hand, asks security verify-cert to use the default verification policy, basic, so it will omit the following certificate.

Cert 1: mkcert development CA
   Number of trust settings : 1
   Trust Setting 0:
      Policy OID            : SSL
      Result Type           : kSecTrustSettingsResultTrustRoot

Finally, the cgo path is checking if any policy (ssl or any other explicitly set) has a kSecTrustSettingsResult value (ignoring the defaults, see above), with the last one in the array winning, omitting the following certificate (!!).

Cert 1: mkcert development CA
   Number of trust settings : 2
   Trust Setting 0:
      Policy OID            : SSL
      Result Type           : kSecTrustSettingsResultTrustRoot
   Trust Setting 1:
      Policy OID            : Code Signing
      Result Type           : kSecTrustSettingsResultDeny

And I didn't even get into allowed errors.

It's fairly late in the freeze, but I'm inclined to fix these, and maybe even backport them, because ignoring the policy types can lead to inclusion of roots that are not supposed to be trusted for TLS, and although crypto/x509 is not TLS-specific, it is meant to serve the WebPKI. @agl agree?

@gopherbot please open the backport tracking issues.

[katiehockman]

Approved since this is a security patch. Please follow the instructions at https://github.com/golang/go/wiki/MinorReleases to create the cherrypick CL.

I will defer to @FiloSottile as to which CLs should be backported.

[dmitshur]

Moving to Go 1.10.6 milestone as this isn't ready for 1.10.5.

[FiloSottile]

I want this to get tested in beta before backporting it, so I expect it will not reach 1.10, but only 1.11 later on.